Communication equipment that involves orthogonal modulation and demodulation may generate orthogonal errors in processes in an orthogonal modulator and an orthogonal demodulator due to incomplete analogue circuits. The orthogonal errors include a gain error, which represents a difference in gain between I and Q channels, and a phase error, which represents a phase difference between the I and Q channels other than 90°.
An improved design accuracy of an analogue circuit to reduce such orthogonal error generally increases the circuit size and power consumption, and thus cannot be readily built in a battery-driven mobile terminal, which has a limited space. Thus, orthogonal error correction technology by digital signal processing is preferably applied to mobile terminals.
A typical conventional technique that corrects orthogonal errors by digital signal processing is described in, for example, in Patent Literature 1. The communication apparatus described in the Patent Literature 1 supplies locally-generated signals having different phases in each other to an orthogonal modulator and an orthogonal demodulator. The communication apparatus then separates an orthogonal error in the orthogonal modulator from that in the orthogonal demodulator, and estimates and corrects the individual errors, by means of the dependency of signal-point mapping on phase rotation. Patent Literature 1 discloses a method for solving a multi-dimensional non-linear equation through digital signal processing for separating the orthogonal error in the orthogonal modulator from that in the orthogonal demodulator.